The present invention relates to telephone equipment and more particularly to a new and improved bell ringing device which is capable of causing any line or combination of lines of a multiple button key telephone set to ring without producing interference in any other line.
In the typical multiple button key telephone set, it is desirable that all incoming lines or any combination of lines be made to ring to signal incoming calls without causing interference in any other line. In the past, this has sometimes been accomplished by common bell or matrix half-cycle ringing. It has also been proposed in patent No. 3,406,262 to Grandstaff to use the ringing current in any one of a plurality of telephone lines to energize a glow lamp the radiation from which impinges on a photocell and initiates operation of a common electronic ringer.
A similar, common, audible signaling circuit utilizing radiation from a neon lamp falling on a photocell to generate an initiating signal in response to ringing current is disclosed in patent No. 3,875,348 to Kopec et al. All such proposals require wiring and electrical components external to the telephone instrument and, therefore, are time consuming and expensive to install.